1. Field of the Invention
This invention relates to a mechanism for electrostatically chucking an object and, more particularly, to a mechanism for holding a substrate in a system for processing the substrate under a vacuum atmosphere.
2. Description of the Related Art
A technique for electrostatically holding an object is frequently used with vacuum processing systems for processing substrates under a vacuum atmosphere.
FIG. 4 is a sectional schematic front view to show a configuration example of an electrostatic chucking mechanism adopted for a plasma enhanced chemical vapor deposition (PECVD) system as an example of a conventional electrostatic chucking mechanism together with the configuration of the system.
The electrostatic chucking mechanism shown in FIG. 4 comprises an electrode body 1 and a dielectric block 2 being placed on the front of the electrode body 1 and containing a dielectric portion 21 dielectrically polarized by a DC voltage applied via the electrode body 1 for electrostatically chucking an object to be chucked.
First, the electrode body 1 is a member shaped like a low circular cylinder or prism and formed of a metal such as aluminum.
The dielectric block 2 is a member shaped like a disk or flat thin square piece projecting on the substrate chucking side for chucking a substrate 3 on the surface of the projecting portion. The dielectric block 2 is formed of ceramics or the like, consisting essentially of aluminum oxide. In the following description, the side chucking the substrate 3 will be referred to as the "front" and the opposite side thereto as the "rear".
The electrode body 1 is fixed on its front to the rear of the dielectric block 2 by using screws. That is, the electrode body 1 is formed with through-holes that fixing screws 4 threadably engage, and the fixing screws 4 pass through the through-holes and project forwardly. Small recesses are made in positions corresponding to the projecting positions of the fixing screws 4 on the rear of the dielectric block 2 and metal tap bodies 5 are disposed so as to be inserted into the small recesses.
The tap bodies 5 are provided because the dielectric block 2 is hard to be directly threaded; the inner face of the cylindrical member of each tap body is threaded. The fixing screws 4 are screwed in the tap bodies 5, whereby the dielectric block 2 is fixed to the electrode body 1.
A bias high-frequency power supply 6 for applying a bias voltage to the substrate 3 by the interaction with plasma is connected to the electrode body 1 via a blocking capacitor 61. A DC power supply 7 for electrostatic chucking is also connected to the electrode body 1 via a filter 71 for removing high frequency components. Further, a flow path 11 for allowing a heating medium (gas or liquid) for temperature adjustment to flow is formed in the electrode body 1, and pipes 12 and 13 are provided for supplying and collecting the heating medium to and from the flow path.
On the other hand, an auxiliary electrode plate 22 is embedded in the dielectric block 2 in a parallel position with the front face of the dielectric block 2. The auxiliary electrode plate 22 and the electrode body 1 are electrically connected to each other by a connection block 23 made of a conductor. A DC voltage applied from the DC power supply 7 to the auxiliary electrode plate 22 causes the dielectric portion 21 on the top of the auxiliary electrode plate 22 to be dielectrically polarized, whereby the substrate 3 is electrostatically chucked.
The PECVD system adopting the above-described electrostatic chucking mechanism consists mainly of a reactor 100 in which the electrostatic chucking mechanism is placed, an exhaust channel 101 for exhausting air from the inside of the reactor 100, a plasma chamber 102 located communicating with the space in the reactor 100, a gas introduction mechanism 103 for introducing reaction gases into the reactor 100, and a power supply mechanism 104 for energizing the gases diffused in the plasma chamber 102 for generating plasma. In addition, a member 105 is provided for electrically insulating the electrode body 1 from the reactor 100, and teflon, quarts, alumina ceramics or the like is usually used for the member 105.
The PECVD system shown in FIG. 4 introduces, for example, mono-silane gas, oxygen gas, and argon gas by the gas introduction mechanism 103 and supplies a high-frequency power by the power supply mechanism 104. The mono-silane gas decomposes in the generated plasma and a silicon oxide thin film deposits on the substrate 3 chucked by the electrostatic chucking mechanism. At this time, the bias high-frequency power supply 6 operates and interacts with the plasma for applying a bias voltage to the substrate 3, whereby ions in the plasma come into collision with the substrate 3 and energy of the ion collision aids in growth of the thin film.
The electrostatic chucking mechanism as described above is often used in thermally harsh environments. For example, with the electrostatic chucking mechanism adopted for the PECVD system described above, the electrode body is maintained at about 100.degree. C. by a heating medium allowed to flow in the electrode body during the PECVD processing. Further, ion collision caused by a bias voltage causes a substrate to be heated to about 400.degree.C., for example, and therefore the temperature of the electrostatic chucking mechanism also is heated to such a temperature.
On the other hand, since plasma does not exist in the break period between completion of PECVD processing of one substrate and processing of another substrate, any bias voltage is not applied either and therefore the temperature of the electrostatic chucking mechanism is restored to about 100.degree. C. When another substrate is transferred in the reactor and PECVD processing is started, again the electrostatic chucking mechanism is heated to about 400.degree. C. This means that the temperature of the electrostatic chucking mechanism rises and falls repeatedly between 100.degree. C. and 400.degree. C.
If the electrostatic chucking mechanism is thus used under a thermally harsh condition that the temperature of the electrostatic chucking mechanism vigorously rises and falls repeatedly in a high-temperature region, the following problem arises: From the thermal expansion coefficient difference between the dielectric block 2 made of ceramics or the like, and the metal members of the electrode body 1, the tap bodies 5, the fixing screws 4 or the like, thermal stress distortion concentrates on the peripheries of the fixing screws 4 and the dielectric block 2 may sometimes be cracked in the peripheries of the fixing screws 4.
To avoid this problem, it is possible to use an adhesive for bonding the dielectric block 2 to the electrode body 1. However, in doing so, as the temperature rises, the organic components of the adhesive evaporate and adhere to the substrate, causing the substrate to be contaminated. A problem that the adhesive degrades due to evaporation of the organic components and that the bonded part will come off may also arise.